A lift truck typically is a battery powered vehicle having an operator compartment with controls that enable the operator to drive the truck and to hoist materials and carry them quickly throughout a factory or warehouse. An upright telescopic mast is attached to the forward end of the truck and with a carriage, or forks, supporting materials can be hoisted by extending the telescopic mast upward.
An exemplary lift truck is shown in FIGS. 1 and 2. It includes an operator compartment 10, a battery 11 and outriggers, or baselegs, 12A and B. A three section, telescopic mast 20 attaches to the front of the truck and includes a base section 21 and two telescopic sections 22 and 23. As shown best in FIG. 2, the lower telescopic section 22 (referred to in the art as the “outer” telescopic section) is nested within the base section 21 and the higher telescopic section 23 (referred to in the art as the “inner” telescopic section) is nested inward of the outer telescopic section 22.
A fork carriage 13 is slidable mounted to the inner telescopic section 23 and it is moved up and down thereon by carriage free lift cylinders 13A and B via chains 13C which pass over pulleys 13D. The outer telescopic section 22 is moved relative to the base section 21 by a main lift cylinder 22A located midway between the left and right mast sections. Lift chains (not shown in FIGS. 1 and 2) fastened to the base section 21, extending over pulleys at the top of the outer telescopic section 22, and fastened to the bottom end of the inner telescopic section 23 provide a simultaneous and coordinated movement of the inner telescopic section 23 relative to the outer telescopic section 22. Operation of the main lift cylinder 22A using controls in the operator compartment 10 may thus extend or contract the two telescopic sections 22 and 23. Operation of the carriage free lift cylinders 13A and B from the operator compartment 10 also controls the precise height of the fork carriage 13.
These mast elements plus the associated hydraulic hoses and electrical cable provide obstructions which limit the operator's field of view when looking forward towards the forks from the operator compartment 10. This is particularly true when the mast is lowered and all the cylinders 22A, 13A and 13B are disposed directly in front of the operator.
Many efforts have been made to improve the operators' field of view when looking forward through the mast. These include shortening the main lift cylinders as disclosed in U.S. Pat. Nos. 4,191,276 and 4,261,438 so that it does not obstruct view when the mast is lowered, shifting the location of the main lift cylinder to one side as disclosed in U.S. Pat. No. 4,355,703; shifting the location of the single main lift cylinder to one side and shifting a single carriage free lift cylinder to the other side as disclosed in U.S. Pat. No. 4,506,764; and shifting the location of the two carriage free lift cylinders to locations nearer the mast uprights to increase visibility as described in U.S. Pat. Nos. 4,369,861; 4,365,693; 4,030,568 and 4,441,585. Yet another approach disclosed in U.S. Pat. No. 4,585,093 is to locate the two carriage free lift cylinders substantially behind the mast uprights and provide two main lift cylinders which are also behind the respective mast uprights. This is carried one step further in U.S. Pat. No. 6,505,710 in which the two main lift cylinders are formed into the base section of the mast.
A significant constraint on the design of a lift truck mast structure is its fore to aft dimension. The length of a lift truck is a very important characteristic, since turning radius is directly related to length. The productivity of a truck and operator is directly related to the turning radius since in the tight confines of factories and warehouses a smaller turning radius translates to less back-and-forth jockeying of the truck. The elimination of one or more inches in the length of a truck therefore has significant economic significance.